1. Field of the Invention
This invention relates to a conductive polymer used for an electrode material in an electrochemical cell such as a secondary battery, an electric double-layer capacitor, a redox capacitor and a condenser, as well as an electrochemical cell therewith. In particular, it relates to an electrode material whereby charge/discharge cycle properties can be improved without deterioration in an apparent capacity or input/output properties, and an electrochemical cell therewith.
2. Description of the Related Art
There have been suggested and practically used secondary batteries, electric double-layer capacitors, redox capacitors and condensers in which a proton-conducting compound is used as an electrode active material. FIG. 1 is a schematic cross-sectional view showing a typical example of such an electrochemical cell.
The electrochemical cell shown in FIG. 1 has a configuration where a cathode 2 and an anode 3 containing a proton-conducting compound as an active material are formed on a cathodic collector 1 and an anodic collector 4, respectively, these electrodes are combined via a separator 5 and only protons are involved as a charge carrier. The cell is filled with an aqueous or non-aqueous solution containing a proton-donating electrolyte after ionization, i.e., a proton source, as an electrolytic solution, and is sealed by a gasket 6.
The cathode 2 and the anode 3 can be prepared using an electrode material containing a doped or undoped proton-conducting compound powder, a conduction auxiliary and a binder. These electrodes can be formed by a method comprising the steps of placing the electrode material in a mold with a predetermined size and molding it by a hot press to form a solid electrode, or a method comprising the steps of depositing an electrode material slurry on a conductive substrate by screen printing and drying the resulting film to form an electrode. Then, a cathode 2 and an anode 3 thus formed can be mutually faced via a separator 5 to give an electrochemical cell.
Examples of a proton-conducting compound used as an electrode active material include π-conjugated polymers such as polyaniline, polythiophene, polypyrrole, polyacetylene, poly-p-phenylene, polyphenylene-vinylene, polyperinaphthalene, polyfuran, polyflurane, polythienylene, polypyridinediyl, polyisothianaphthene, polyquinoxaline, polypyridine, polypyrimidine, polyindole, polyaminoanthraquinone, polyimidazole and their derivatives; indole π-conjugated compound such as an indole trimer compound; quinones such as benzoquinone, naphthoquinone and anthraquinone; quinone polymers such as polyanthraquinone, polynaphthoquinone and polybenzoquinone where a quinone oxygen can be converted into a hydroxyl group by conjugation); and proton-conducting polymer prepared by copolymerizing two or more of the monomers giving the above polymers. These compounds may be doped to form a redox pair for exhibiting conductivity. These compounds are appropriately selected as a cathode and an anode active material, taking a redox potential difference into account.
Known electrolytic solutions include an aqueous electrolytic solution consisting of an aqueous acid solution and a non-aqueous electrolytic solution comprising an electrolyte in an organic solvent. The former aqueous electrolytic solution is exclusively used because it can give a high-capacity cell. The acid used may be an organic or inorganic acid; for example, inorganic acids such as sulfuric acid, nitric acid, hydrochloric acid, phosphoric acid, tetrafluoroboric acid, hexafluorophosphoric acid and hexafluorosilicic acid and organic acids such as saturated monocarboxylic acids, aliphatic carboxylic acids, oxycarboxylic acids, p-toluenesulfonic acid, polyvinylsulfonic acid and lauric acid.
Japanese Laid-open Patent Publication Nos. 2000-156329, 2001-143748 and 1995-320780 have disclosed specific examples of an electrolytic solution and an electrolyte used in a common electrochemical cell.
Japanese Laid-open Patent Publication No. 2000-156329 has disclosed an electrolytic solution for an aluminum electrolytic condenser comprising a quaternary salt formed from a quaternary cation of a compound having an N,N,N′-substituted amidine group and an organic acid anion as a solute. Examples of a compound having an N,N,N′-substituted amidine group include cyclic amidines having an imidazole ring, a 2-imidazoline ring or a tetrahydropyrimidine ring. Examples of an organic acid cation include those of carboxylic acids, and mono and dialkyl phosphates. Furthermore, it has described that the electrolytic solution can improve heat resistance, increases a specific conductance, improves durability and minimize deterioration of a sealer.
Japanese Laid-open Patent Publication No. 2001-143748 has disclosed a non-aqueous electrolytic solution for a lithium secondary battery containing a lithium salt of a perfluoroalkylsulfonic acid derivative and a heterocyclic compound having at least one fluorine atom and nitrogen or oxygen. Examples of the heterocyclic compound include pyrazoles, imidazoles, triazoles, oxazoles and coumarins. It has also described that in the electrolytic solution, the heterocyclic compound can prevent a cathode collector from being oxidatively deteriorated and improve charge/discharge cycle properties.
Japanese Laid-open Patent Publication No. 7-320780 has disclosed the use of polyamides, polyimidazoles, polyimides, polyoxazoles, polytetrafluoroethyele, polymelamine formaldehyde, polycarbonates and polypropylene as an electrolyte for a solid electrolyte secondary battery. It has also described that using the polymer can prevent the electrolyte from reacting an anode and improve charge/discharge cycle properties.
We have filed Japanese Patent Application No. 2003-198660 (Japanese Laid-open Patent Publication No. 2004-127920) disclosing that a nitrogen-containing heterocyclic compound or a polymer comprising a unit having a nitrogen-containing heterocycle structure is used as an electrode active material for preventing an electrode active material from being deteriorated by over-oxidation or over-reduction and for improving charge/discharge cycle properties of a secondary battery. Examples of these compounds described includes imidazole, triazole, pyrazole and their derivatives, and polymers comprising a unit having any of these structures.
The invention disclosed in Japanese Laid-open Patent Publication No. 2004-127920 is based on the observation that a nitrogen-containing heterocyclic compound incorporated in an electrode material can prevent an electrode active material from being deteriorated by over-oxidation or over-reduction. In other words, in this technique, a nitrogen-containing heterocyclic compound is added to and mixed with an electrode or a polymer having a structure of a nitrogen-containing heterocyclic compound, specifically a copolymer having an imidazole moiety is used as a material for an electrode, in order to improve charge/discharge cycle properties of a secondary battery.
Further specific examples are a polyphenylquinoxaline represented by formula (13) as a comparative example, and a copolymer of benzimidazole and phenylquinoxaline represented by formula (14), which act as an active material. In the copolymer represented by formula (14), as a content of the structural unit of the nitrogen-containing heterocyclic compound increases, charge/discharge cycle properties are improved while an electron conductivity is reduced, in comparison with the polymer represented by formula (13).
In particular, when the structural unit of the nitrogen-containing heterocyclic compound is 50 wt % or more to the structural unit of the polyphenylquinoxaline, the content is so high that an electron conductivity is significantly reduced and a reaction resistance with ions in an electrolytic solution is increased, leading to deterioration in charge/discharge cycle properties.

This problem would be due to the chemical structure of the polymer, probably the following reasons. The chemical structure of the copolymer represented by formula (14) is a structure where the framework of the polymer main chain is a random polymer structure of the phenylquinoxaline structural unit and the benzimidazole structural unit, in which as a chemical composition, only a weight ratio of the phenylquinoxaline structural unit to the benzimidazole structural unit is controlled. In other words, regularity of an alignment of the structural units is not controlled in terms of a chemical structure.
Such a copolymer in which regularity of an alignment of the structural units is not controlled has a structure where two different structural units are randomly arranged, so that in comparison with a conventional polymer having one structural unit, i.e., a homopolymer, regularity in a chemical structure is reduced and formation of a conduction path for electrons is hindered, leading to tendency to considerable reduction in an electron conductivity.
For example, the phenylquinoxaline-benzimidazole copolymer represented by formula (14) has a structure where a benzimidazole moiety is introduced in the polyphenylquinoxaline framework with good electron conductivity because of its long π-conjugated system and thus breaks the conjugated system. Specifically, in comparison with the polyphenylquinoxaline (formula 13), formation of a conduction path for electrons is hindered, resulting in a reduced electron conductivity of the copolymer (formula 14).
In theory, an electron conductivity is significantly influenced by a length of a π-conjugated system and a density of a carrier for electrons. Thus, it is believed that introduction of a benzimidazole moiety into a polymer may reduce an electron conductivity because a longer π-conjugated system and a higher carrier density give a higher conductivity.
Therefore, an electron conductivity of a copolymer having such a chemical structure is significantly influenced by a weight ratio of a structural unit having an imidazole moiety; as the weight ratio increases, an electron conductivity is significantly reduced.
Furthermore, an electrochemical cell using such a copolymer as an electrode material has an increased internal resistance, leading to a reduced energy and deterioration in quick charge/discharge properties, and, when a content of the structural unit of the nitrogen-containing heterocyclic compound is excessive, deterioration in charge/discharge cycle properties.